Reabsorption of bile acids and lecithin has been demonstrated in rabbit and guinea pig gallbladders. The relative contribution of reabsorption to the failure of micellar solubilization of cholesterol in gallstone-forming bile is not known. We propose to extend our previous studies of bile acid reabsorption a) to the owl monkey, a gallstone susceptible species and, b) to the intrahepatic bile duct system, on the theory that reabsorption of micellar constituents may also occur there. Ductal reabsorption is postulated to explain how bile supersaturated with cholesterol routinely appears after an overnight fast. If, as seems plausible, cholesterol is normally excreted in micellar solution, supersaturation would be impossible unless micellar elements were lost. Reabsorbed bile acids might also inhibit bile acid synthesis. The permeability in the ductal system will be measured by retrograde infusion of labelled bile acids under known pressure into the ducts of one liver lobe while monitoring the effluent bile for radioactivity in a widely separated lobe. Similarly, forward perfusion of labelled bile acids by way of a peripheral ductule with simultaneous collection of bile from another lobe will be carried out. Distention of the gallbladder has been found to induce marked bile acid reabsorption in the rabbit. A small ballooned catheter will be used to monitor intravesicular pressure in unanesthetized owl monkeys during 24-hour feeding cycles. Since we found alternate day feeding caused enlarged gallbladders and enhanced gallstone formation in several species, the effect of this alternate starving and stuffing on intravesicular pressure will be compared with the effect of a normal feeding frequency. In addition, at various pressures the reabsorptive capacity of the owl monkey gallbladder for lecithin and several bile acids will be tested. A pilot study in rabbits has shown cholic acid escapes from the gallbladder more readily than chenodeoxycholic acid. The beneficial effect of chenodeoxycholic acid on cholesterol solubilization may be due to its preferential retention in the gallbladder and bile ducts.